1. Field of the Invention
The present invention relates to a high-speed rotating shaft of a supercharger.
2. Description of Related Art
Previously compressing an air or an air-fuel mixture supplied to a cylinder of an internal combustion engine is called as a supercharging, and a compressor thereof is called as a supercharger. Further, a supercharger executing the supercharging by utilizing an exhaust gas of the engine is called as an exhaust gas turbine supercharger or a turbocharger for short. In the following description, in the present application, the turbocharger is simply called as “supercharger” except a particularly necessary case.
FIG. 1 is a general structure view showing an example of a conventional turbocharger. In this drawing, the turbocharger is constituted by a turbine rotor shaft 1, a compressor impeller 2, a bearing housing 3, a turbine housing 4, a compressor housing 5a, a seal plate 5b and the like.
The bearing housing 3, the turbine housing 4, the compressor housing 5a and the seal plate 5b are coupled to each other in an illustrated order. Further, the turbine rotor shaft 1 is formed by integrating a turbine impeller 1a and a rotor shaft 1b in accordance with a welding or the like, is rotationally supported by a radial bearing within the bearing housing 3, and is coaxially coupled to the compressor impeller 2.
In accordance with this structure, it is possible to widely improve a performance of the internal combustion engine by rotationally driving the turbine impeller 1a by the exhaust gas of the internal combustion engine, transmitting a rotating force to the compressor impeller 2 via the rotor shaft 1b so as to rotationally drive the compressor impeller 2, and compressing the air (or the air-fuel mixture) so as to supply to the internal combustion engine.
In FIG. 1, the rotation of the turbine impeller 1a is supported in a radial direction by two floating metals 6a and 6b, and is supported in a thrust direction by a turbine side thrust bearing 8a and a compressor side thrust bearing 8b via a thrust collar 7. In this case, in this drawing, reference numeral 9 denotes an oil thrower, and reference symbol 6c denotes a bearing spacer.
In accordance with a high performance of the supercharger, the turbine rotor shaft 1 and the compressor impeller 2 are rotated at a high speed between several tens of thousand and several hundreds of thousand min-1. The floating metals 6a and 6b rotate at lower speed than the turbine rotor shaft because these metals are not fixed with the shaft, and the thrust collar 7 rotate at the same speed as that of the turbine rotor shaft because it is fixed with the shaft. Therefore the floating metals 6a and 6b and the thrust collar 7 support the radial force and the thrust force respectively while rotating with respective high speed. Further, in order to reduce a sliding resistance at a time of rotating, the structure is made such that a lubricating oil is always supplied to the sliding portion from an oil path 3a provided in the bearing housing 3.
Further, as a bearing structure of the turbine rotor shaft rotating at a high speed, patent documents 1 to 3 have been already disclosed.
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-110577 “bearing apparatus of supercharger”
Patent Document 2: Japanese Unexamined Patent Publication No. 2001-295655 “bearing apparatus of supercharger”
Patent Document 3: Japanese Unexamined Patent Publication No. 2005-23920 “bearing apparatus of supercharger”
As mentioned above, the high-speed rotating shaft (the turbine rotor shaft) of the conventional supercharger is normally supported by two radial bearings spaced at a fixed distance. In this case, a specific frequency ω of the high-speed rotating shaft can be expressed by an approximate expression (1) in the case of the high-speed rotating shaft except the turbine and the compressor in both ends.ω=(π/2)×(n/L)2×(EI/ρA)0.5  (1)
In this case, reference symbol n(=1, 2, 3, . . .) denotes a vibration mode degree (primary, secondary and tertiary) of a both-end support shaft shown in FIGS. 2A to 2C, reference symbol L denotes a bearing center distance, reference symbol E denotes a longitudinal elastic modulus, reference symbol I denotes a moment of inertia of the cross section of the high-speed rotating shaft, reference symbol ρ denotes a density, and reference symbol A denotes a cross sectional area.
Further, if a diameter of the high-speed rotating shaft is set to d, an expression (2) can be obtained from the expression (1) on the basis of I=πd4/64, A=πd2/4, and (I/A)0.5=d/4.ω=(π/2)×(n/L)2×(d/4)×(E/ρ)0.5  (2)
In this case, each of the expressions mentioned above corresponds to the approximate expression, and it is practically necessary to determine a critical speed in accordance with a strict computer simulation or the like including the turbine and the compressor in both ends.
Hereinafter, the rotating speed corresponding to the primary, secondary and tertiary vibration modes is called as “critical speed of bending” or simply called as “critical speed”, in the present invention.
The high-speed rotating shaft of the conventional supercharger is designed such that the secondary critical speed of the shaft is sufficiently away from the rated speed which means maximum design speed. In such design, the primary critical speed becomes higher. Therefore when the rotating speed of the shaft passes through the primary critical speed, excitation energy applied to the supercharger becomes bigger and vibration and noise becomes larger.
Further, as shown in the drawing of the patent document 1, in order to improve a rotational stability of the supercharger, it is possible to decrease the primary critical speed of the high-speed rotating shaft by narrowing a distance between the bearings. Accordingly, for example, in the case that the rotating speed of the high-speed rotating shaft passes through the primary critical speed during the operation of the supercharger from a low-speed rotation to a high-speed rotation, there has been executed a reduction of a vibration and a noise by reducing an excitation energy applied to the supercharger.
However, as is apparent from the expression (2), it is generally possible to decrease the primary critical speed by narrowing a shaft diameter between the bearings, however, the secondary critical speed tends to be simultaneously decreased. Accordingly, there is a problem that the secondary critical speed is lowered largely in some shaft system so as to come close to the operation range and the shaft system becomes unstable.